Conventionally, a fuel cell system capable of high-efficiency and small-scale power generation easily constructs a system for making use of heat energy generated in power generation, and therefore has been developed as a distributed power generation system capable of achieving high energy use efficiency.
During a stop operation of the fuel cell system, when the interior of the system is cooled to have a negative pressure, and thereby air flows into the system, residual hydrogen inside the fuel cell may be rapidly oxidized by oxygen in air, and the fuel cell system may be damaged due to a reaction heat resulting from the oxidation reaction.
To solve such a problem, a fuel cell system has been proposed, in which a residual gas inside the fuel cell is purged using a raw material such as a natural gas and the raw material is filled in a sealed state inside the fuel cell, at the time of stop operation (e.g., patent document 1).
FIG. 10 is a view of a configuration of the proposed fuel cell system. As shown in FIG. 10, the conventional fuel cell system includes a fuel gas generator 500 configured to generate a hydrogen-rich fuel gas from a raw material which is mainly composed of carbon and hydrogen, a fuel gas supply path 502 through which the fuel gas is supplied from the fuel gas generator 500 to a fuel cell 501 and an off-gas supply path 503 through which a fuel gas (hereinafter referred to as off-gas) which has not been used in power generation and is exhausted from the fuel cell 501 is supplied to a combustion burner 500a of the fuel gas generator 500. The fuel cell system further includes a fuel cell bypass path 504 provided between the fuel gas supply path 502 and the off-gas supply path 503 to switch a fuel gas destination from the fuel cell 501 to the combustion burner 500a of the fuel gas generator 500, a material supplier 505 configured to supply a raw material used to generate the fuel gas to the fuel gas generator 500, and a fuel gas generator bypass path 506 for directly feeding the raw material from the material supplier 505 to the fuel cell 501 so as to bypass the fuel gas generator 500. A three-way valve 507 is provided at a branch portion where the fuel gas supply path 502 branches into the fuel cell bypass path 504. In addition, an on-off valve 508 is provided at a portion of the off-gas supply path 503 in a location upstream of a junction portion where fuel cell bypass path 504 is joined to the off-gas supply path 503. A blower 509 is provided to supply air as an oxidizing gas to the fuel cell 501.
In the above proposed fuel cell system, at the time of the start-up operation, the fuel gas which is generated in the fuel gas generator 500 and contains carbon monoxide having a higher concentration than carbon monoxide at the time of the power generation, is supplied to the combustion burner 500a of the fuel gas generator 500 through the fuel cell bypass path 504. The combustion gas is combusted in the combustion burner 500a to heat a reforming catalyst and increase its temperature.
After the start of the power generation operation, when the temperature of the reforming catalyst in the fuel gas generator 500 reaches a predetermined temperature, the fuel gas generated in the fuel gas generator 500 is supplied to the fuel cell 501 through the fuel gas supply path 502. The fuel gas is used as a fuel for power generation in the fuel cell 501. The off-gas which is exhausted from the fuel cell 501 is supplied to the combustion burner 500a of the fuel gas generator 500 via the off-gas supply path 503. In the combustion burner 500a, the off-gas is combusted to heat the reforming catalyst.
In the proposed fuel cell system, after stop of the power generation operation of the fuel cell system, the raw material is fed as a replacement gas from the material supplier 505 and filled into the fuel cell 501 via the fuel gas generator bypass path 506. Thereby, during a stop state of the power generation operation of the fuel cell system, the raw material such as the natural gas, which replaces an inert gas such as a nitrogen gas, is filled in a sealed state inside the fuel cell 501 and its surrounding region.
The raw material such as the natural gas which is fed from the material supplier 505 and filled into the fuel cell 501 at the time of the stop operation is chemically stable as compared to the hydrogen contained in the fuel gas. Therefore, even if air is mixed with the raw material such as the natural gas remaining inside the fuel cell 501 during the stop state of the power generation operation, a rapid oxidation will not proceed.
As should be appreciated from the above, in the conventional fuel cell system, damage to the fuel cell system due to the reaction heat generated by the oxidation reaction is effectively prevented by filling the raw material such as the natural gas into the fuel cell 501.    Patent document 1: Japanese Laid-Open Patent Application Publication 2003-229149